The powder metallurgy technology has many advantages over traditional metalworking production: cost, energy, and material efficiency, high volume capability, net shape forming without machining, etc. However, sintered metal articles often rank below cast and wrought metals in some physical and mechanical properties. Relatively low density is usually the main drawback of sintered metals that results in insufficient properties of production, and retards a wider distribution of the powder metallurgy technology. Another problem is the inability to provide a radial flow of the porous metal in open dies in the second pressing operation. High density of sintered articles is absolutely necessary in such structurally sensitive applications as magnetic materials, particularly in articles sintered from iron-silicon powder alloys.
Many of these iron-silicon sintered articles have an intricate design that is characterized by a combination of thin and thick intersections, local radial changes of the shape, and tight dimensional tolerances. A uniform distribution of the density and the chemical composition in the volume of articles is also required. The specific shape of these iron-silicon articles requires the use of precise tooling for powder compacting (die and punch) that will not withstand pressure much higher than 50 tsi and allows to reach the maximum green density over 6.8 g/cc (88% of theoretical value). On the other hand, those iron-silicon sintered articles are in mass production and their manufacturing should not be expensive. Therefore, the cold pressing on automatic presses with rigid dies is the most preferable process for the iron-silicon powder compaction. Further densification of the article body to the value over 7.4 g/cc should occur during the high temperature sintering in order to achieve the required properties.
Various techniques have been developed to increase the density of powdered articles to near theoretical. "Double press double sintering" method is the most popular among them. But this method is not applicable to iron-silicon powder alloys because the hardness of the sintered article is so high after the first cycle of sintering, that any significant densification in cold pressing during the second molding cycle is impossible and no radial material flow may be achieved.
Pure iron and low carbon steel powders for powder metallurgy are used in the production of sintered parts usually having a green density of 6.0-7.2 g/cc (76-92% of the theoretical value). These parts are being made by mixing iron powder with copper and graphite powders, and the like, shaping into a green compact in a mold, sintering, and if necessary, sizing a sintered body for dimensional correction.
However, the sintered body produced by adding copper powder, graphite powder or the like to the iron powder is high in the strength, so that the drawback is that the dimensional correction can not be conducted to a satisfactory extent due to spring-back of the sintered body even if the sizing for the dimensional correction is made.
But, as we have stated above, any reasonable densification in sizing after the sintering is impossible for iron-silicon articles for reason of their hardness and brittleness. Besides, any copper and/or graphite admixtures to the iron-silicon composition cannot be tolerated because they significantly deteriorate all magnetic properties.
Most other conventional densification methods, such as Hot Isostatic Pressing, warm pressing, preliminary pulverization of powders, or special pre-alloying of iron powder, are also not suitable for the production of soft magnetic iron-silicon articles because some of these methods cause excessive manufacturing expenses and others contaminate the magnetic material with impermissible impurities.
The process of producing high density sintered alloys as disclosed in the U.S. Pat. No. 5,516,483 comprises a blending of initial powders such as compressible iron powder, carbon, ferro alloys and lubricant, pressing said blended mixture to the shape in a single compacting stage. Then, the formed article can be sintered at a temperature higher than 1300.degree. C. The single step of the compaction obtained in this proposed method occurs between 6.5 to 6.8 g/cc. Authors of this invention reported the final density of the article being as high as 7.3 g/cc and even 7.7 g/cc. This is the result of separate grinding of ferro alloy powders to a particle size 8-12 microns and of sintering at a temperature that is significantly higher than in other conventional methods. It means that the shrinkage during the sintering provides the densification from 0.5 to 0.9 g/cc. This high densification in the sintering stage adversely affects the final tolerances of article sizes, especially for complicated shape parts.
Such extreme densification at the cost of sintering only is impossible for iron-silicon compacted alloys consisting of silicon more than 3 wt. % because the liquid-phase sintering of these alloys occurs only during the first minutes of the sintering process at the constant temperature. As the interaction between solid iron powder and silicon-rich liquid phase is being developed, the liquidus temperature of the molten alloy drastically increases (as it is known from previous trials and clearly demonstrated on the Fe--Si Phase Diagram). Further processing occurs as the solid-phase sintering is characterized by substantially lower shrinkage. Moreover, the master alloy, that is usually used in the initial blend for the production of iron-silicon magnetic articles, has about 31 wt. % of silicon and consists of more than 50% of .epsilon.-phase (FeSi intermetallics) that is melted only at a temperature greater than 1410.degree. C. In other words, the liquid-phase sintering is never realized completely in the temperature range of 1250-1380.degree. C. that is used for sintering according to U.S. Pat. Nos. 5,516,483, 5,540,883, and 5,476,632. We need to obtain substantially greater green density of iron-silicon compacted articles, for instance 7.0-7.1 g/cc instead of 6.5-6.8 g/cc, in order to reach the final density level over 7.4 g/cc (96% of theoretical value) after sintering. Besides, the higher green density will provide better dimensional tolerances of the articles after their sintering. But the method described in U.S. Pat. Nos. 5,516,483, 5,540,883, and 5,476,632 as well as all conventional methods designed in other referred patents do not offer a solution how to reach the density greater than 6.8 g/cc after cold pressing iron-silicon articles having intricate shapes.
Accordingly, it is the principal object of the present invention to provide a method of producing iron-silicon soft magnetic sintered articles of intricate design which articles exhibit a uniform density greater than 88% of the theoretical value (6.8 g/cc) before the sintering and the final density of sintered articles greater than 96% of the theoretical value (7.4 d/cc).
A further objective of this invention is to provide a cost effective method for producing iron-silicon sintered articles having a density level that is mentioned above by means of automatic cold presses and a single sintering operation.